Update index.html

index.html

@@ -422,6 +422,7 @@
         <ul>
           <li><a href="#">Overview</a></li>
           <li><a href="http://xr.thereisonlyxul.org/" target="_blank">Cross-Reference</a></li>
+          <li><a href="http://files.thereisonlyxul.org/" target="_blank">Files</a></li>
           <li><a href="https://github.com/thereisonlyxul" target="_blank">GitHub</a></li>
         </ul>
       </div>